Polyurethane sealant compositions typically comprise at least one urethane prepolymer. Adhesives useful for bonding to non-porous substrates, such as glass are described, for example, in U.S. Pat. No. 4,374,237 and U.S. Pat. No. 4,687,533, both incorporated herein by reference. Window installation in a vehicle is typically a three-stage process. First, a clear silane primer is applied to the glass to clean and prepare the surface for bonding. Second, a primer, which is essentially a carbon black dispersion which also contains a compound having silane and/or isocyanate functionality, often referred to as “black-out primer”, is then applied over the top of the clear primer. Third, an adhesive is applied to the primed glass which is then installed into tire structure. For vehicles and some buildings, these materials are applied to a frit of a ceramic enamel or an organic coating located about the periphery of the window. The frit is designed to protect the adhesive from exposure to UV light and to hide the adhesive and/or trim components from view, see, U.S. Pat. No. 5,370,905, incorporated herein by reference. Further, when such compositions are used to bond glass substrates to painted substrates, such as for instance, window installation in vehicle manufacturing, the lap shear strength of the bonded substrate may be less than desirable for safety or structural purposes. Consequently, a separate paint primer comprising a solution of one or more silanes and/or isocyanate compounds is often applied to a painted substrate prior to the application of the composition in most vehicle assembly operations for bonding the windshield and the rear window. Alkoxy silane moieties have been reacted into the backbone or onto a portion of the end groups of isocyanate functional prepolymers so as to enhance the adhesion of compositions containing such prepolymers to substrates such as glass and coated surfaces. See Wu U.S. Pat. No. 6,512,033B1; Berger et al. U.S. Pat. No. 4,374,237 and Rizk et al. U.S. Pat. No. 4,687,533 incorporated herein by reference.
In order for an adhesive system to be commercially viable, that system must provide a durable bond. Wu U.S. Pat. No. 6,512,033 discloses the addition of additives which enhance the durability of cured compositions containing such prepolymers. “Durable bond” means that the adhesive holds the window into the structure for a period of years. As the structure to which window glass or plastic coated with an abrasion resistant coating is traditionally bonded lasts for a significant number of years, it is expected that the bond holding the glass or coated plastic into a structure also last a significant number of years. Durable bond preferably means that the adhesive composition once cored maintains its bond to substrates for up to 10 to 20 years when exposed to normal use conditions. Durability is often predicted by accelerated aging tests. In one context durability means that the adhesive exhibits acceptable performance in an accelerated aging test, such as exposing the bonded substrate to water at 90° C. for a period of time, exposure to weatherometer conditions for an extended period over 1000 hours and more preferably over 2000 hours or cataplasma exposure. The additives disclosed in Wu U.S. Pat. No. 6,512,033 can add significant expense to the adhesive compositions.
Adhesive systems utilized in bonding glass into structures often contain fillers. Fillers are added for a variety of reasons including to reduce the cost of the adhesive, to add strength or to color the adhesive. The problem is that if too much filler is added, the inherent properties of the adhesive can be compromised. The conventional wisdom is that adhesive compositions containing isocyanate functional prepolymers containing alkoxy silane moieties may exhibit poor stability. Stability as used in this context means that the adhesive in storage and prior to cure does not undergo unacceptable viscosity growth. If the viscosity of the adhesive composition grows too much the adhesive cannot be applied to a substrate using standard application equipment. It is believed that the presence of fillers, especially calcium carbonate, can negatively impact the stability of adhesive compositions.
It would be desirable to provide a composition which contains fillers which provides a bonded structure with lap shear strengths which meet industry standards, exhibits low viscosity growth during storage and which allows for durable adhesion of the composition to the substrate surfaces, especially where low amounts or no durability stabilizers are utilized.